The present invention is directed to stretch wrap films and methods for their use. In particular, the present invention is directed to stretch wrap films having excellent maximum stretch, moderate holding force, superior puncture resistance, high total energy dart drop, high cling force and overall strength.
The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads is a significant commercially important application of polymer film, including generically, polyethylene and other polyolefins.
Over-wrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned on a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching or tensioning force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common.
At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Another wrapping method finding acceptance in industry today is that of hand wrapping. In this method, the film is again arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The operator may install the roll of film so used on a hand-held wrapping tool for ease of use.
Some of the properties desired of a good stretch wrap film are as follows: good cling or cohesion properties, high puncture resistance, good machine direction tear resistance, good transparency, low haze, low stress relaxation with time, high resistance to transverse tear especially when under machine direction tension, producible in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, and low modulus of elasticity.
Currently, different grades of stretch wrap films are commonly marketed for different end uses according to overall film properties. For example, certain stretch wrap films have superior properties for load retention, but these films are characterized by having poor stretching characteristics. On the other hand, certain stretch wrap films having superior stretching properties have low load retention properties, thus limiting their use. Some of the broader categories include: general purpose stretch films, premium or heavy duty stretch films, single sided cling films, general purpose hand wrap films, heavy duty hand wrap films, and special formulation hand wrap films.
A need exists to develop superior general purpose stretch wrap films characterized by having moderate load retention characteristics and excellent stretching characteristics while still maintaining other important stretch film properties. Such films could be used in a wider variety of end applications and, thus, not unduly limit users of stretch wrap films to selectively choosing a film based on its properties prior to initiating a stretch wrap application.
Accordingly, the present invention provides for multi-layer films for general purpose stretch applications including stretch wrap films. The preferred resultant film has five layers with each layer being a single component or blended components to achieve the desired improved results.
In accordance with the present invention, attention has been focused on developing higher performance stretch films, while maintaining cost efficient production capabilities. Performance enhancements include improvement of load retention, tear resistance in machine direction (MD) and transverse direction (TD), puncture resistance, ultimate elongation, and overall strength. Desired properties of a "good" stretch film are good cling or cohesion properties, good tear resistance in MD and TD directions, good clarity (low haze), high ultimate elongation, high tensile values, good stiffness (modulus), and high yield per pound.